1. Field of the Invention
The present invention relates to a projection optical system for forming an image of a first plane on a second plane, an exposure apparatus incorporating this projection optical system and used at the time of transferring a mask pattern onto a substrate in a lithography process for manufacturing devices or micro devices, such as semiconductor devices or liquid-crystal display devices, and a manufacturing method for micro devices using this exposure apparatus to manufacture micro devices such as semiconductor devices, imaging devices, liquid-crystal display devices or thin film magnetic heads.
2. Description of the Related Art
When micro devices such as semiconductor devices are manufactured, there are used a batch exposure type projection exposure apparatus (stepper or the like) in which a minute pattern image formed on a reticle as a mask is transferred onto a wafer (or a glass plate) on which a resist is applied, via a projection optical system, or a scanning exposure type projection exposure apparatus involving a step and scan method. In order to form a minute pattern on a wafer or the like, it is necessary to increase the resolution of a pattern image of the reticle obtained by irradiating an illumination light onto the reticle.
As a method of increasing the resolution of the pattern image, there is a method involving making the illumination light mainly emitted from a light source a short wavelength, and a method involving designing the numerical aperture (NA) of the projection optical system to be high. However, even if the numerical aperture of the projection optical system is increased over and above what is required, if illumination light having a long wavelength is used, there is naturally a limit to improvement of the resolution. Therefore, it is basically necessary to make the wavelength of the illumination light short. Heretofore, as the wavelength of illumination light, a g ray (436 nm) or an i ray (365 nm) has been often used. At present, however, the wavelength of illumination light has been made shorter, and a laser beam (248 nm) emitted from a KrF excimer laser or a laser beam (193 nm) emitted from an ArF excimer laser is now being used. Moreover, a projection optical system which can be used under exposure light having such a short wavelength is under development.
In evaluating the performance of the projection optical system, it is important to have a high numerical aperture, since this becomes an index for obtaining a high resolution. However, even if the numerical aperture is high, if aberration occurs, there is a problem in forming minute patterns. Here, description is made of chromatic aberration, as one example of aberration. Since the optical performance of a projection optical system used in applications for forming minute patterns is quite high, it becomes necessary to make each aberration substantially zero. In order to make chromatic aberration zero, the projection optical system has been heretofore realized by a dioptric system constituted by only lens groups. This method however, requires a multiplicity of lenses, and hence invites a reduction in transmittance, and an increase in the cost for manufacturing the projection optical system cannot be avoided. Moreover, as is generally known, the condition for making curvature aberration of an image surface zero is to satisfy the Petzval's condition. In order to satisfy this condition however, it is necessary to combine not only lenses having a positive power but also lenses having a negative power. As a result, this is not desirable in view of improving transmittance and cost reduction.
As compared with dioptric optical elements such as lenses, in the diffractive optical elements, the reaction of chromatic aberration is in a direction opposite to that for the normal dioptric optical element. Therefore, by merely mixing the dioptric optical elements and the diffractive optical elements, chromatic aberration can be corrected. Moreover, since the diffractive optical element since power can be set to a predetermined value without making any contribution to the Petzval's condition, it is possible to make the image surface flat, designating curvature aberration as zero, without causing an increase in the number of the dioptric optical elements. Also, since the diffractive optical element can optionally set the angle of diffraction, this has an advantage in that it can be prepared as an optical element having a similar action to that of an aspherical lens.
As described above, by using the diffractive optical element, chromatic aberration can be corrected and curvature aberration can be made zero, without causing an increase in the number of the dioptric optical lenses. This result is especially suitable for improving transmittance of the projection optical system and reducing the cost. A technique adopting such an optical system combining the diffractive optical element and the dioptric lens for the projection optical system for semiconductor manufacturing apparatus has been disclosed in, for example, Japanese Unexamined Patent Application, First Publication Nos. Hei 1-307443, Hei 4-214516, Hei 6-331941, Hei 7-128590 and Hei 8-17719.
Below is a description of optical characteristic of the diffractive optical element. At the time of using the diffractive optical element, it is preferred to use a phase-type diffractive optical element (kinoform), in view of diffraction efficiency, and in view of ease of production the cross-section of the diffractive optical element should preferably be in a saw-tooth pattern (blazed type) or a stepwise pattern (binary optical element). Diffraction efficiency in this case stands for an intensity ratio between light incident on the diffractive optical element and diffracted light after predetermined order. With the diffractive optical element having a cross-section in a saw-tooth pattern or a stepwise pattern, occurrence of unnecessary diffracted light which does not contribute to image forming cannot be avoided due to form error or the like. However, as disclosed in, for example, Japanese Unexamined Patent Application, First Publication No. Hei 11-307443, when this unnecessary diffracted light has an intensity ratio and an intensity distribution of a desired numerical value or below, the influence thereof on the imaging performance can be substantially ignored.
When the wavelength of illumination light becomes short with shortening of the wavelength of the light source, the kinds of usable glass material are limited due to absorption of light, and if the wavelength becomes 180 nm or below, the only glass material practically usable is fluorite. Therefore, under illumination light having such a short wavelength, correction of chromatic aberration becomes impossible with a construction having only dioptric lenses. Hence it is necessary to perform correction of chromatic aberration, using a diffractive optical element.
Moreover, when the exposure wavelength becomes ultraviolet light, it is necessary to form the ring width (pitch) around the diffractive optical element to be very small in order to obtain effective power for correcting the chromatic aberration, using the diffractive optical element. Hence the production thereof becomes difficult. In such a case, a diffractive optical element which obtains the power of the diffractive optical element within a range capable of production and reduces an occurrence of unnecessary diffracted light as much as possible, and a projection optical system using this diffractive optical element are disclosed in Japanese Unexamined Patent Application, First Publication Nos. Hei 5-150107, Hei 5-297209 and Hei 6-331941.
However, for a projection optical system for forming a pattern having a resolution of 0.1 μm or less, using extreme ultraviolet illumination light having a large numerical aperture, measures have not heretofore been taken for exhibiting sufficient imaging performance, taking into consideration incident angle characteristics and manufacturing error of the diffractive optical element.
In view of the above situation, a first object of the present invention is to provide a projection optical system that can form an image of a first plane on a second plane under suitable conditions, using extreme ultraviolet illumination light. Also, a second object of the present invention is to provide a projection optical system that can achieve the first object without causing a cost increase. Moreover, a third object of the present invention is to provide an exposure apparatus comprising such a projection optical system, which can form a minute pattern of 0.1 μm or less on a wafer arranged on the second plane, and to provide a manufacturing method for micro devices, using this exposure apparatus.